January 07, 2006

Redundant Digital File Transfer and Ham Digital Radio Mondiale

You can find DSSTV (Digital Slow Scan Television) usually on the SSTV (Slow Scan Television) Calling frequency of 144.500mhz in the local area of Warrington and the Northwest of England.

G2ARV and M0JGM along with many other regulars can be found using the Digitrx software using either of the RDFT or HAM DRM modes.

A little about the creator of RDFT:

Indiana resident Barry Sanderson, KB9VAK, is being named as recipient of the Technical Excellence Award for the development of a multi-channel, multiphase modulation scheme known as Redundant Digital File Transfer (RDFT). Not only did Barry ‘do the math’ but he also wrote the core software routines that allow RDFT to run on personal computers using sound card DSP capabilities. This allows error-free transmission of computer files via standard amateur radio equipment. This technology has been proven very powerful in the area of digital slow-scan television.

A little about DRM:

For those who don’t know much about it Digital Radio Mondiale, DRMâ„¢ for short, is a method of broadcasting on AM radio using digital signals rather than analogue – it’s like listening to an audio stream on the Internet, but broadcast on medium or shortwave radio rather than via the net.

In addition to potentially crystal-clear reception, DRMâ„¢ signals can also carry multimedia information in addition to the audio signal, or to carry multiple program streams on one signal:

Used mainly on the HF bands it is also used on 2m with much success.

PY4ZBZ
The HamDRM carriers uses OFDM: ortogonal frequency division multiplex.
The modulated carriers are overlapping in frequency domain and are
modulated in QAM (amplitude and phase).
In the current state of RDFT, the transmission of a file needs three
separate operations : Coding, Recording (of the transmission) and Decoding,
and this increases the total time of transmittin/receiving.
In HamDRM, a file is codified and transmitted simultaneously.
And it is decoded at the same time in that is received. There is only a
small delay between the end of the transmission and the reception
(some seconds).
To transmit a file needs no more separate coding and decoding as in RDFT.
Just load the file (if not to big), or use Jp2 converter for adjust the
picture file a suitable size in bytes.
The file is divided in blocks (RDFT) or segments (HamDRM).
The size of segments depends on the mode particularities.
The Mode A is faster than Mode B but it doesn’t support negative effects
caused by multi-path propagation as Mode B, which is more robust.
The QAM modulation can be in 4 or 16 or 64 vectors or symbols.
64 QAM is much faster but needs a very good signal to noise ratio (SNR),
or a high quality channel.
4 QAM is the slowest but it supports more noise, or smaller SNR.
Interleaving consists of changing the natural symbols sequence.
If long (2 seconds), it allows better error correction with slow fading,
but it causes a little larger delay in the reception.
Short interleaving = 400 ms.
The occupied band width BW for the HamDRM signal can be set to 2,3 or
2,5 kHz. The smallest is a little slower and it should be used if the
SSB TX/RX equipments filters are to narrow or have not a flat
transfer function.
The DC frequency simply refers to the inferior limit of BW, and also
should be chosen in agreement with the equipments filters.
The amount of instances is the number of times that a file is transmitted
during one transmission. More than 1 instance allows that if during the
reception of the first instance some segments were not received, they
will probably be received during the second or third instance,
correcting the received file.
Leadin is just a extra time of initial transmission so that the receiver
can have time to synchronize correctly.
BSR is an adapted system from BBR of DIGTRX and allows to do a request
to send only for the missing segments of a received file.